Juice production and the yield thereof from fruits and vegetables can be understood by looking at the different components involved in the process. The components important for the present invention are pectin, protopectin, calcium ions and pectinases.
Pectins are major constituents of the cell walls of edible parts of fruits and vegetables. The middle lamella which are situated between the cell walls are mainly built up from protopectin which is the insoluble form of pectin. Pectins are considered as intercellular adhesives and due to their colloid nature they also have an important function in the water-regulation of plants. Water-binding capacity is greatly increased by the amount of hydrophylic hydroxyl and carboxyl groups. The amount of pectin can be very high. For example, lemon peels are reported to contain pectin up to 30% of their dry weight, orange peels contain from 15-20% and apple peels about 10% (Norz, K., 1985. Zucker und S usswaren Wirtschaft 38 5-6).
Pectins are composed of a rhamno-galacturonan backbone in which 1,4-linked .alpha.-D-galacturonan chains are interrupted at intervals by the insertion of 1,2-linked .alpha.-L-rhamnopyranosyl residues (Pilnik, W. and A. Voragen 1970. In `The Biochemistry of fruits and their products`, Vol. 1, Chapter 3, p.53. Acad. Press). Other sugars, such as D-galactose, L-arabinose and D-xylose, are present as side chains. A large part of the galacturonan residues is esterified with methyl groups at the C2 and C3 position. Apple pectin is methylated to a degree of about 90% (Aspinall, G. and Fanous, 1984. Carbohydrate Polym. 4 p. 193). Pectin exist in two forms, pectin and protopectin. Protopectin is pectin which is insoluble in water and strongly bound to the cell wall (Joslyn, M. 1962, Food Research Vol. 11, p.1-107. Acad. Press)
In fruit such as apples, during the maturation an increase of the soluble pectin content and a decrease of the non-soluble protopectin content can be observed. The release of apple pectin is due to a partial solubilization of the protopectin from the lamella. This solubilization or pectinolysis is due to the endogenous pectinases present in the fruit. As a result of this process the amount of pectin in the juice after pressing increases whereas the amount of protopectin left in the pomace decreases with storage time of the fruit.
Calcium ions are present in free form in the apple pulp or bound to the pectic acid in the form of calcium pectate which increases the firmness of the pulp. (Walkinshaw, M. and S. Arnott, 1981. J. Mol. Biol. 153 p.1075). After harvesting, and at least partially due to pectinolysis and dehydrating of the apples, the bound calcium is released and migrates into the apples. This release of calcium further increases protopectin solubilization (Perring, A. et al. 1985. J. Sci. Food Agric. 36 p. 1035). At the same time this process leads to a decrease of the pulp firmness.
Both the endogenous apple pectinases and the endogenous calcium have a major effect on the residual amount of protopectin in the pomace. The apple pomace contains a high amount of protopectin at the beginning of the season, which decreases due to the combined action of endogenous pectinase activities and calcium migration. This is the major reason why the pectin manufacturer prefers the use of pomace obtained early in the harvesting season in order to secure a high process yield.
Apple or citrus pectins are used in the preparation of food for their gelating capacity. Apple pectin is used for jelly or jam production since it gelifies under acidic conditions in the presence of sugar.
In the classical juice production process, apple juice is obtained after grinding and pressing which separates the liquid phase from the residual solids. In pectin-rich fruit such as apple, mechanical crushing gives an apple juice with a high viscosity which increases with the maturation time of the fruit. Furthermore, a substantial part of the juice remains in the pulp in the form of a gel. After prolonged storage of the apples it becomes more and more difficult to extract the juice from the pulp by pressing or other mechanical methods due to the increase in the amount of soluble pectin. Therefore, the apple juice yield decreases with time. The apple juice obtained after enzymatic maceration and pressing is depectinized with exogenous pectinases, clarified by fining agents or ultra-filtration, pasteurized and bottled or concentrated by evaporation under vacuum. The pomace which is a leftover of the apple juice preparation can subsequently be used for pectin extraction. In this process no maceration enzymes are employed.
In an improved juice production process the juice yield is substantially increased by adding exogenous pectinases to the apples before pressing (Endo A., 1965. Agric. Biol. Chem. 29(2) p. 137). The pectinase preparations most widely used for fruit maceration are capable of rapid degradation of highly esterified pectins (Voragen, A. and W. Pilnik 1989. In `Biocatalysis in Agricultural Biotechnology`, ACS Symposium series 389, chapter 7, p.93). The pectinase preparations generally consist of a mixture of activities. The activities in these mixture are dependent on the source from which they are obtained. Based on enzymatic activity the pectinases are classified in two groups: the saponifying enzymes (pectin methyl esterase PME and pectin acetyl esterase PAE) and the depolymerase enzymes which digest the polygalacturonic chains. The enzymes involved in the second group are a pectin lyase (PL) and polygalacturonases (PG) (exo or endo).
The use of these enzymes is dependent on the substrate, the degree of methylation and the molecular weight of the pectin. PE and PG or PL first act to solubilize the protopectin in pectin and subsequently hydrolyze the pectin from the cell wall, thereby at the same time releasing the juice contained inside the cell vacuola.
The increased pulp pressability and the increase in juice yield are due to the fact that degraded pectins have lost their capacity to bind water. The enzymatic treatment improves the juice yield of apples which are difficult to press and also from apples stored for longer periods of time.
Pectin hydrolysis stimulated by the addition of exogenous pectinases gives a rapid decrease in viscosity, an improvement of the pulp pressability, a disintegration of the jelly structure and a higher apple juice yield. However, the exogenous pectinases consisting of a mixture of enzymatic activities (such as PE, PGand PL) at the same time release and degrade the main part of the protopectin which is bound in the pomace. The pomace cannot be used economically anymore for the production of pectin. Furthermore, this stimulated enzymatic degradation of pectin gives rise to an increase in degradation products like uronic acid, or more generally, oligogalacturonides in the juice. These degradation products cause undesired non-enzymatic browning during storage (Voragen A. et al., 1988. Z.Lebensm. Forsch. 187 p.315-320).
Summarising, it can be concluded that in the classical juice preparation procedure the juice yield is relatively low, the juice is cloudy and the amount of pectin which can be extracted from the pomace is high. Introduction of pectinases lead to an increased juice yield, a lower protopectin content of the pomace and unwanted side-reactions giving rise to browning of the juice.
Clearly there is a need for an enzyme preparation which combines the advantages of an increase in apple juice yield and improved pressability with the absence of unwanted side-reactions of the enzymes and the lack of undesired hydrolysis of the protopectin which is present in the cell walls.